1. Field of the Invention
The present invention relates to a production method of highly pure pyromellitic dianhydride which is used as materials for highly heat-resistant polyimide resins, cross-linking agents for foamed polyesters, specialty plasticizers, etc.
2. Description of the Prior Art
Pyromellitic acid is known to be produced by the liquid-phase oxidation of durene and the liquid-phase oxidation of 2,4,5-trimethylbenzaldehyde, etc. It has been also known that the crude pyromellitic acid obtained in these methods can be converted into pyromellitic dianhydride by the anhydrization in the presence of an aliphatic acid anhydride such as acetic anhydride (for example, Japanese Patent 2515296). The method proposed in the patent requires two mole of acetic anhydride per one mole of pyromellitic acid and necessitates an additional treatment of by-produced acetic acid, increasing the production costs. In another method, pyromellitic acid is converted into pyromellitic dianhydride by the dehydration under heating at limited temperatures (for example, JP 62-59280A). This method is disadvantageous because of its difficulty in controlling the particle properties and preventing the discoloration of pyromellitic dianhydride. Also known is a vapor-phase oxidation of durene or 2,4,5-trimethylbenzaldehyde (for example, JP 2000-1484A). The pyromellitic dianhydride produced by this method contains a small amount of by-produced monoanhydrides such as trimellitic anhydride. Such monoanhydrides should be removed as completely as possible, because they act as the polymerization inhibitor of pyromellitic dianhydride and diamine for the production of polyimide resins. JP 2001-59022A proposes linear or acicular particles of pyromellitic dianhydride having an angle of repose of 50 to 70°, which is used to increase the molecular weight of thermoplastic resins. However, these techniques cannot control the particle size of pyromellitic dianhydride particles, and the proposed pyromellitic dianhydride particles cause problems of plugging, etc. in industrial use because of their high angle of repose.
The crude pyromellitic acid generally contains impurities such as by-product of oxidation reaction and intermediate compounds. Of such impurities, aromatic dicarboxylic acids such as phthalic acid and aromatic tricarboxylic acids such as trimellitic acid and methyltrimellitic acid, which are converted into monoanhydrides simultaneously with the anhydrization of pyromellitic acid, should be removed as completely as possible. Pyromellitic monoanhydride, which is formed by the incomplete or partial anhydrization of pyromellitic acid, should be also removed as completely as possible. Since the melting point is as extremely high as 287° C., pyromellitic dianhydride is generally used in particle forms. The pyromellitic dianhydride proposed in the above patent documents can cause plugging of pipelines, outlets of silos, inlets of reactors, etc. because of its particle properties. If pyromellitic dianhydride is discolored, thermoplastic resins produced using pyromellitic dianhydride as a raw material or an additive may be also discolored. The above problems can be avoided in some extent by the anhydrization of pyromellitic acid by acetic anhydride under heating. However, an increased amount of acetic anhydride should be required, and additionally, acetic acid and acetic anhydride after use should be removed from the system together with pyromellitic dianhydride dissolved therein to prevent the accumulation of impurities in the system, increasing production costs.